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PANNA: Pesticides and Amphibians

Strong evidence is emerging that reproduction and development in aquatic animals is threatened by pesticides. A new study, released jointly by researchers at the Canadian Wildlife Service, the Redpath Museum at McGill University, and the Ontario Veterinary College explores this connection further.

The researchers surveyed frog and toad populations in agricultural areas in the St. Lawrence Valley, Canada, for developmental defects such as limb deformities. They compared amphibians that live in agricultural areas where pesticides are applied regularly to those that live in areas where little pesticide use has occurred. Many frogs had extra legs growing from their stomachs and backs; other frogs had only stumps for hind legs or fused hind legs. Some frogs were missing eyes or had extra eyes; others were missing toes or had extra ones. The scientists examined some frogs for internal effects and found enlarged and diseased livers, as well as female organs inside frogs that otherwise appeared to be male.

After observing nearly 30,000 frogs and toads, they determined that the incidence of limb deformities in animals undergoing metamorphosis between tadpole and frog is 7% overall in agricultural habitats, but only 1.5% in the non-agricultural areas. The prevalence of deformed tadpole- frogs was between zero and 67% in pesticide-contaminated sites but only between zero and 7.7% in the control sites. The deformity rate of adult frogs from agricultural areas was 2.6%, just slightly higher than that for unexposed population at 1.5%. This indicates that many of the deformed tadpole- frogs did not survive to adulthood, a fact that is likely responsible for the overall drop in frog and toad populations in the area. Canada has 45 frog and toad species and of these 17 are in decline, largely due to human activities that destroy habitat or contaminate water.

One of the researchers, Dr. Martin Oulette, is convinced that the damage is due to pesticides. On a farm near St. Charles in the St. Lawrence Valley, every frog he found was deformed. "We have to know why the frogs are deformed and why they are dying," says Oulette. "We're also living in the St. Lawrence Valley and we put the food coming from there on our tables."

Researchers have been investigating deformities in amphibians for a number of years. In 1994, Florida alligators were shown to have been feminized by endocrine-disrupting pesticides that interfere with normal development. In many species, including amphibians, fish and humans, the endocrine and thyroid hormones control the process of development. Exposure to hormone-mimicking substances such as many commonly used pesticides results in interrupted development, as in the case of the Florida alligators, where the incidence of deformed reproductive organs was extremely high due to exposure to the pesticide dicofol. Because amphibians live in the water, it is clear that they are the "front line" when it comes to exposures.

More needs to be known about endocrine-disrupting substances before we can fully evaluate their potential for environmental damage. Scientists already suspect a link between endocrine disruptors such as PCBs and declining sperm counts, testicular cancer and genital defects in human males. The World Wildlife Fund recently published a paper entitled "Chemicals the Compromise Life: A Call to Action" highlighting work on endocrine-disrupting chemicals that has taken place since the "Our Stolen Future" was published in 1996 (see PANUPSResource Pointer #184).

A comprehensive list of the chemicals that are responsible for these problems is not yet available nor is there adequate information about the specific effects of these chemicals on different species. Based on the recommendations of the Endocrine Disruptor Screening and Testing Advisory Committee (EDSTAC) released in September 1998, the U.S. Environmental Protection Agency is implementing an extensive testing program to screen approximately 62,000 chemicals for their endocrine-disrupting potential by the year 2000. The two- tiered screening plan gives high priority to chemicals with widespread exposure at the national level, as well as those that cause high exposures in certain groups,communities or ecosystems.